Re: [PATCH v9 2/8] KVM: Extend the memslot to support fd-based private memory

Chao Peng
2022-10-31T14:19:19Z

On Fri, Oct 28, 2022 at 03:04:27PM +0800, Xiaoyao Li wrote:
> On 10/25/2022 11:13 PM, Chao Peng wrote:
> > In memory encryption usage, guest memory may be encrypted with special
> > key and can be accessed only by the guest itself. We call such memory
> > private memory. It's valueless and sometimes can cause problem to allow
> > userspace to access guest private memory. This new KVM memslot extension
> > allows guest private memory being provided though a restrictedmem
> ^
>
> typo

Thanks!

>
> > backed file descriptor(fd) and userspace is restricted to access the
> > bookmarked memory in the fd.
> >
> > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> > additional KVM memslot fields restricted_fd/restricted_offset to allow
> > userspace to instruct KVM to provide guest memory through restricted_fd.
> > 'guest_phys_addr' is mapped at the restricted_offset of restricted_fd
> > and the size is 'memory_size'.
> >
> > The extended memslot can still have the userspace_addr(hva). When use, a
> > single memslot can maintain both private memory through restricted_fd
> > and shared memory through userspace_addr. Whether the private or shared
> > part is visible to guest is maintained by other KVM code.
> >
> > A restrictedmem_notifier field is also added to the memslot structure to
> > allow the restricted_fd's backing store to notify KVM the memory change,
> > KVM then can invalidate its page table entries.
> >
> > Together with the change, a new config HAVE_KVM_RESTRICTED_MEM is added
> > and right now it is selected on X86_64 only. A KVM_CAP_PRIVATE_MEM is
> > also introduced to indicate KVM support for KVM_MEM_PRIVATE.
> >
> > To make code maintenance easy, internally we use a binary compatible
> > alias struct kvm_user_mem_region to handle both the normal and the
> > '_ext' variants.
> >
> > Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> > Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> > Documentation/virt/kvm/api.rst | 48 ++++++++++++++++++++++++++++-----
> > arch/x86/kvm/Kconfig | 2 ++
> > arch/x86/kvm/x86.c | 2 +-
> > include/linux/kvm_host.h | 13 +++++++--
> > include/uapi/linux/kvm.h | 29 ++++++++++++++++++++
> > virt/kvm/Kconfig | 3 +++
> > virt/kvm/kvm_main.c | 49 ++++++++++++++++++++++++++++------
> > 7 files changed, 128 insertions(+), 18 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index eee9f857a986..f3fa75649a78 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -1319,7 +1319,7 @@ yet and must be cleared on entry.
> > :Capability: KVM_CAP_USER_MEMORY
> > :Architectures: all
> > :Type: vm ioctl
> > -:Parameters: struct kvm_userspace_memory_region (in)
> > +:Parameters: struct kvm_userspace_memory_region(_ext) (in)
> > :Returns: 0 on success, -1 on error
> > ::
> > @@ -1332,9 +1332,18 @@ yet and must be cleared on entry.
> > __u64 userspace_addr; /* start of the userspace allocated memory */
> > };
> > + struct kvm_userspace_memory_region_ext {
> > + struct kvm_userspace_memory_region region;
> > + __u64 restricted_offset;
> > + __u32 restricted_fd;
> > + __u32 pad1;
> > + __u64 pad2[14];
> > + };
> > +
> > /* for kvm_memory_region::flags */
> > #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
> > #define KVM_MEM_READONLY (1UL << 1)
> > + #define KVM_MEM_PRIVATE (1UL << 2)
> > This ioctl allows the user to create, modify or delete a guest physical
> > memory slot. Bits 0-15 of "slot" specify the slot id and this value
> > @@ -1365,12 +1374,27 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
> > be identical. This allows large pages in the guest to be backed by large
> > pages in the host.
> > -The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
> > -KVM_MEM_READONLY. The former can be set to instruct KVM to keep track of
> > -writes to memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to
> > -use it. The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
> > -to make a new slot read-only. In this case, writes to this memory will be
> > -posted to userspace as KVM_EXIT_MMIO exits.
> > +kvm_userspace_memory_region_ext struct includes all fields of
> > +kvm_userspace_memory_region struct, while also adds additional fields for some
> > +other features. See below description of flags field for more information.
> > +It's recommended to use kvm_userspace_memory_region_ext in new userspace code.
> > +
> > +The flags field supports following flags:
> > +
> > +- KVM_MEM_LOG_DIRTY_PAGES to instruct KVM to keep track of writes to memory
> > + within the slot. For more details, see KVM_GET_DIRTY_LOG ioctl.
> > +
> > +- KVM_MEM_READONLY, if KVM_CAP_READONLY_MEM allows, to make a new slot
> > + read-only. In this case, writes to this memory will be posted to userspace as
> > + KVM_EXIT_MMIO exits.
> > +
> > +- KVM_MEM_PRIVATE, if KVM_CAP_PRIVATE_MEM allows, to indicate a new slot has
> > + private memory backed by a file descriptor(fd) and userspace access to the
> > + fd may be restricted. Userspace should use restricted_fd/restricted_offset in
> > + kvm_userspace_memory_region_ext to instruct KVM to provide private memory
> > + to guest. Userspace should guarantee not to map the same pfn indicated by
> > + restricted_fd/restricted_offset to different gfns with multiple memslots.
> > + Failed to do this may result undefined behavior.
> > When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
> > the memory region are automatically reflected into the guest. For example, an
> > @@ -8215,6 +8239,16 @@ structure.
> > When getting the Modified Change Topology Report value, the attr->addr
> > must point to a byte where the value will be stored or retrieved from.
> > +8.36 KVM_CAP_PRIVATE_MEM
> > +------------------------
> > +
> > +:Architectures: x86
> > +
> > +This capability indicates that private memory is supported and userspace can
> > +set KVM_MEM_PRIVATE flag for KVM_SET_USER_MEMORY_REGION ioctl. See
> > +KVM_SET_USER_MEMORY_REGION for details on the usage of KVM_MEM_PRIVATE and
> > +kvm_userspace_memory_region_ext fields.
> > +
> > 9. Known KVM API problems
> > =========================
> > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > index 67be7f217e37..8d2bd455c0cd 100644
> > --- a/arch/x86/kvm/Kconfig
> > +++ b/arch/x86/kvm/Kconfig
> > @@ -49,6 +49,8 @@ config KVM
> > select SRCU
> > select INTERVAL_TREE
> > select HAVE_KVM_PM_NOTIFIER if PM
> > + select HAVE_KVM_RESTRICTED_MEM if X86_64
> > + select RESTRICTEDMEM if HAVE_KVM_RESTRICTED_MEM
> > help
> > Support hosting fully virtualized guest machines using hardware
> > virtualization extensions. You will need a fairly recent
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 4bd5f8a751de..02ad31f46dd7 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12425,7 +12425,7 @@ void __user * __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa,
> > }
> > for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> > - struct kvm_userspace_memory_region m;
> > + struct kvm_user_mem_region m;
> > m.slot = id | (i << 16);
> > m.flags = 0;
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 32f259fa5801..739a7562a1f3 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -44,6 +44,7 @@
> > #include <asm/kvm_host.h>
> > #include <linux/kvm_dirty_ring.h>
> > +#include <linux/restrictedmem.h>
> > #ifndef KVM_MAX_VCPU_IDS
> > #define KVM_MAX_VCPU_IDS KVM_MAX_VCPUS
> > @@ -575,8 +576,16 @@ struct kvm_memory_slot {
> > u32 flags;
> > short id;
> > u16 as_id;
> > + struct file *restricted_file;
> > + loff_t restricted_offset;
> > + struct restrictedmem_notifier notifier;
> > };
> > +static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
> > +{
> > + return slot && (slot->flags & KVM_MEM_PRIVATE);
> > +}
> > +
>
> We can introduce this function in patch 6 when it's first used.

Good to me.

Chao
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